Sodium azide poisoned, detergent treated Bacillus subtilis cells lose 60 percent of their cellular proteins including DNA polymerase-1 and nucleases. The chromosome and its replicating systems are still intact and can promote sequential, semiconservative DNA synthesis using externally added deoxynucleoside triphosphates. The system also requires ATP for normal DNA synthesis. In its absence only repair synthesis is observed. We plan to use this system that produces biologically active DNA in vitro for analyzing biochemical aspects of replication, repair and recombination of DNA. The various enzymes, initiator proteins and membrane components that are involved in this system will be isolated and characterized by both, biochemical and genetic techniques. In addition, the system is amenable to in vitro complementation studies using various conditional lethal DNA synthesis mutants. A phage system is also being developed to study the mechanism of DNA replication. The information obtained in these systems will be used to study the replication of mammalian chromosomes.